Our most popular heat pack is the 8×18. WARNING may cause extreme relaxation and comfort. This heat pack is designed to reach the shoulders, neck and upper back all at the same time. It can be used to wrap any part of the body. The 8×18 lasts up to three hours depending on how well you insulate it. You can also use the heat pack as a cold pack. Just place it in the freezer for up to one hour and it will stay soft and flexible.